A modern application specific integrated circuit (ASIC) must meet very stringent design and performance specifications. One of the factors that influence the design and performance of an ASIC is the characteristic impedance of the transmission lines that are located on a printed circuit board (PCB), interconnect structure, package, or other elements associated with an ASIC. The terms “impedance” and “characteristic impedance” are used interchangeably. Typically, it is desirable to control the characteristic impedance of a transmission line so as to maximize the efficiency of signal transfer in both a single-ended signal application and in a differential signal application. As its name implies, a single-ended signal is one that is transferred using a single transmission line. A differential signal is one that is represented by two complementary signals on different conductors (i.e., transmission lines), with the term “differential” representing the difference between the two complementary signals. A “differential pair” is a communication methodology that uses two separate conductors (i.e., the differential pair) to carry a differential communication signal. All differential signals also have what is referred to as a “common mode,” which represents the average of the two differential signals. One of the conductors carries a “true” or “positive” version of the differential communication signal while the other conductor carries the “complement” or “negative” version of the differential communication signal.
Regardless of whether the transmission line or lines are adapted to transfer a single-ended signal or a differential signal, the characteristic impedance of the transmission line is important as it relates to the efficiency of signal transfer.
Existing techniques to control the characteristic impedance include, for example, controlling line width, line spacing, dielectric thickness, dielectric constant, and others. In a strip line signal trace, low crosstalk environment, a high characteristic impedance is achieved by reducing line width, increasing dielectric thickness or changing material properties of the trace and/or dielectric, and adjusting the number of layers of material. Unfortunately, for example, reducing line width results in higher insertion loss and increasing the likelihood of signal crosstalk. These existing techniques all have other drawbacks and may not achieve the desired characteristic impedance.
Therefore, it would be desirable to have a way of controlling the characteristic impedance of a transmission line in an ASIC while maintaining a low insertion loss and low likelihood of signal crosstalk.